Field of the Invention
The present invention relates to a method for introducing a getter into a vacuum vessel and for activating the getter in the vacuum vessel, and to a getter unit that is suitable for use in a method of this type.
When, after evacuation, a vacuum vessel is hermetically sealed, the pressure in a vessel of this type tends to rise even if it is impossible for any gas particles to penetrate from the environment into the interior through the vessel walls. The reason for this behavior is gas molecules that are absorbed at the vessel inner walls while the latter is exposed to atmospheric pressure and are released again from the vessel walls in only small quantities and extremely slowly during the evacuation.
To prevent a pressure rise caused by the desorption of such gases in a hermetically sealed vessel, it is known to introduce what are referred to as getter materials, i.e., materials whose surface is able to bond molecules desorbed from the walls of the vacuum vessel significantly more strongly than the vessel walls are able to do. This ability of certain materials is also exploited in ultra-high vacuum technology in what are referred to as getter pumps, the operating principle of which is based on the ionization and electrical acceleration of gas particles that then impinge at high speed on the surface of a getter material, where they are bonded.
For the gettering action, it is always favorable and in many cases even indispensable for the getter material to have been made substantially gas-free prior to the initial uptake of gas. If this does not happen, initially a considerable pressure rise is observed, for example, when a getter pump first starts to operate; this initial pressure rise is attributable to the fact that ions that impinge on the material release large amounts of relatively loosely bonded gas molecules at the surface of this material.
Gettering substances that have taken up gas at relatively high pressures over a prolonged period of time may under certain circumstances act as gas sources at lower pressures, thereby limiting the pressure reduction that can be achieved. Gettering substances that are intended to act alone, i.e., without any additional acceleration of the gas particles that are to be gettered onto the material, therefore, have to be stored packaged in a gastight sleeve in which an atmosphere of a chemically inactive noble gas or vacuum prevails in order to maintain their activity. Before the getter is, then, introduced into a volume that is to be evacuated, it is necessary for the getter to be activated by removal of the protective sleeve. This means that the gettering substance is exposed for a more or less long period of time to a high ambient pressure that saturates its uptake capacity to a greater or lesser extent and, thereby, restricts the activity of the getter. Therefore, when using such getters for the production of products that include a vacuum vessel, it is necessary to keep the time between removal of the getter from the protective sleeve and the evacuation of the vacuum vessel into which the getter has been introduced as short as possible. However, the duration of this period may vary from time to time, and the climatic conditions, in particular, atmospheric humidity and temperature, under which the getter is handled, are also subject to fluctuations over the course of time. Accordingly, the saturation that occurs when the getter is introduced is variable. The amount of getter material that has to be introduced into a vessel that is to be evacuated to enable a predetermined vacuum to be maintained therein for a long period of time, therefore, has to be estimated at a higher level than that corresponding to the nominal, unsaturated uptake capacity of the getter.
In some cases, this represents a very considerable increase in costs.